Before a living cell can reproduce, its DNA strands must unwind from their normal coiled configurations and assume a topology favorable for replication. To allow this unwinding the enzymes known as topoisomerases serve to introduce "swivels" in DNA strands. Without such a mechanism the DNA could not replicate, and hence the cell could not reproduce and proliferate. For detailed explanations of the topoisomerase function see A. Lehninger, Principles of Biochemistry, 813, Worth Publishers, New York (1982); L. F. Liu, "DNA Topoisomerases," CRC Critical Review in Biochemistry, 1-24, 15 (1983) and H Vosberg, "DNA Topoisomerases: Enzymes that Control DNA Conformation," Current Topics in Microbiology and Immunology, 19, Springer-Verlag, Berlin (1985). It has been recognized for some time that cell proliferation might be controlled by inhibition of topoisomerases and that such control might be particularly useful in halting the spread of tumors and related malignancies and ultimately destroying them. See E. Nelson, et al., Proc. Nat. Acad. Sci. U.S.A., 81, 1361 (1984).
On the basis of mechanism of action, topoisomerases have been categorized as Type I and Type II (often referred to as "topo I" and "topo II", respectively). The clinically useful antitumor agents adriamycin, mitoxantrone, etoposide and m-AMSA have been reported to work by inhibiting the function of Type II topoisomerase. Camptothecin, a natural product antitumor agent, has been found to inhibit the function of Type I topoisomerase as have certain synthetic camptothecin analogs (see Wall, et al., U.S. Pat. No. 4,894,456). It is now believed that a compound which could effectively inhibit the functions of either or both Type I and Type II would be a potent antitumor agent.